Advanced membrane electrode assemblies for fuel cells

ABSTRACT

A method of preparing advanced membrane electrode assemblies (MEA) for use in fuel cells. A base polymer is selected for a base membrane. An electrode composition is selected to optimize properties exhibited by the membrane electrode assembly based on the selection of the base polymer. A property-tuning coating layer composition is selected based on compatibility with the base polymer and the electrode composition. A solvent is selected based on the interaction of the solvent with the base polymer and the property-tuning coating layer composition. The MEA is assembled by preparing the base membrane and then applying the property-tuning coating layer to form a composite membrane. Finally, a catalyst is applied to the composite membrane.

RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.11/113,678 entitled “Advanced Membrane Electrode Assemblies for FuelCells,” filed Apr. 22, 2005, now allowed, incorporated by reference.

STATEMENT REGARDING FEDERAL RIGHTS

This invention was made with government support under Contract No.DE-AC52-06NA25396 awarded by the U.S. Department of Energy. Thegovernment has certain rights in the invention.

FIELD OF THE INVENTION

The present invention relates generally to polymer electrolyte membranefuel cells, and, more particularly, to a method for producing advancedmembrane electrode assemblies exhibiting improved performance anddurability.

BACKGROUND OF THE INVENTION

Current fuel cell membrane electrode assemblies (MEAs) suffer fromincompatibilities between the materials used for membranes andelectrodes, especially when non-traditional membranes (e.g., sulfonatedpolysulfone) are used with conventional Nafion®-bonded electrodes. Thesematerial property incompatibilities include differences in water uptake,electro-osmotic drag, and adhesion (chemical composition). The presentinvention method was the result of isolating the factors that affectperformance and durability, and optimizing the method for producingadvanced MEAs taking those factors into account. The present inventionmethod produces MEAs that comprise multiple layers of polymerelectrolytes that exhibit tunable properties leading to improvedperformance, properties, and durability under a wide range of fuel celloperating conditions, to include use in direct methanol and hydrogenfuel cells.

Polymer electrolyte membrane fuel cells (PEMFC) and direct methanol fuelcells (DMFC), a subset of PEMFC, have been the center of attention forover a decade as possible candidates for next generation energyconversion devices. PEMFC and DMFC are currently being developed for anumber of different applications. Some of the most important challengesfor PEMFC construction methods are to reduce the membrane cost, increasedurability, increase the operating temperature range, and increaseconductivity at low levels of relative humidity (RH). The main challengeconcerning DMFCs is to reduce the methanol crossover from anode tocathode, while maintaining high conductivity. Methanol crossoveradversely affects the cell by lowering the cell voltage due to a mixedpotential effect at the cathode (lower power density and efficiency) andlowering fuel utilization (lower efficiency).

The current state of the art perfluorinated sulfonic acid protonexchange membrane, Nafion®, is not only costly but also has a tendencyto creep (limiting its durability, especially at high temperature), poorconductivity under dry conditions, and inherently high methanolpermeability. As a consequence, significant effort has been spentdeveloping alternative hydrocarbon based proton exchange membranes,which are less expensive, have higher glass transition temperatures andlower methanol permeability. Issues involving conduction at low RH arealso being extensively studied, but materials that have adequateconduction for most applications under these conditions have not beenfound. Still, the present invention fabrication method and techniquespresented are applicable to such systems once materials with therequisite properties are developed.

Many polymers [McGrath, et. al, U.S. Patent Application No.20020091225,2002, Koyama et. al, U.S. Pat. No. 6,670,065, 2003, L. Jorissen, et. al.J. Power Sources, 105, 267, 2002, K. Miyatake, et. al. Macromolecules,37, 4961, 2004] have been identified that have promising properties foruse in fuel cell systems, however, use of membranes other thanperfluorinated sulfonic acid polymers have shown little or noperformance improvement in fuel cell testing. In other words,anticipated performance improvements based on membrane properties havenot been realized in functioning devices. A primary barrier to thesuccessful integration of alternative polymeric membranes into highperformance membrane electrode assemblies is attributed to minimizinginterfacial resistance loss and interfacial delamination between themembrane and the electrode under fuel cell operating conditions.

The present invention allows for the incorporation of alternativepolymers in fuel cell systems, while maintaining robust, highperformance membrane electrode assemblies, and improving long-term cellperformance (power density and/or fuel efficiency) and durability.

Therefore, in accordance with the present invention, a membrane coatingfabrication method and consequent fuel cell membrane have been developedto overcome performance degradation arising from interfacial resistancedue to the dimensional mismatch between membrane and electrodematerials. The present invention can be applied to direct methanol andhydrogen fuel cells using proton exchange membrane especially forsituations where the chemical and/or water swelling differences betweenthe electrode and membrane are dramatic.

Various objects, advantages and novel features of the invention will beset forth in part in the description which follows, and in part willbecome apparent to those skilled in the art upon examination of thefollowing or may be learned by practice of the invention. The objectsand advantages of the invention may be realized and attained by means ofthe instrumentalities and combinations particularly pointed out in theappended claims.

SUMMARY OF THE INVENTION

In accordance with the purposes of the present invention, as embodiedand broadly described herein, the present invention includes a method ofpreparing advanced membrane electrode assemblies (MEA) for use in fuelcells. A base polymer is selected for a base membrane. An electrodecomposition is selected to optimize properties exhibited by the membraneelectrode assembly based on the selection of the base polymer. Aproperty-tuning coating layer composition is selected based oncompatibility with the base polymer and the electrode composition. Asolvent is selected based on the interaction of the solvent with thebase polymer and the property-tuning coating layer composition. The MEAis assembled by preparing the base membrane and then applying theproperty-tuning coating layer to form a composite membrane. Finally, acatalyst is applied to the composite membrane.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and form a part ofthe specification, illustrate the embodiments of the present inventionand, together with the description, serve to explain the principles ofthe invention. In the drawings:

FIG. 1 is a flow chart describing the method of the present invention.

FIGS. 2 a and 2 b graphically depict comparative DMFC polarizationcurves using the Nafion® and biphenyl poly(arylene ether sulfone)s after200 and 500 hour fuel cell operation.

FIGS. 3 a and 3 b graphically depict comparative H₂/air polarizationcurves using the Nafion® and biphenyl poly(arylene ether sulfone)s after200 and 500 hour fuel cell operation.

FIGS. 4 a and 4 b schematically depict the layout of a double sided andsingle sided MEA in accordance with the present invention, respectively.

FIGS. 5 a and 5 b graphically depict the DMFC and H₂/air polarizationcurves of alternative hydrocarbon membrane before and after 200 and 500hour fuel cell operation, respectively, in accordance with an embodimentof the present invention.

FIGS. 6 a, 6 b, and 6 c pictorially illustrate how the present inventionallows the ability to tune differing water movement characteristicswithin a subject MEA.

FIGS. 7 a and 7 b graphically depict fuel cell performance and highfrequency resistance of a standard Nafion® MEA and a coated Nafion® MEAas a function of humidification levels and fuel cell operatingtemperatures, respectively, in accordance with an embodiment of thepresent invention.

DETAILED DESCRIPTION

The present invention includes a method for constructing membraneelectrode assemblies (MEAs) for use in fuel cell applications. Invarious embodiments, materials in the construction of MEAs includewholly aromatic polymers based on biphenyl poly(arylene ether sulfone)s(BPSH). These copolymers, with moderate degree of disulfonation, havebeen tested extensively in fuel cells. [Pivovar et. al. TheElectrochemical Society Meeting Abstract, vol. 2001-2, San Francisco,Calif., Sep. 2-7, 2001; Pivovar et. al., AIChE, Fuel Cell Technology:Opportunities and Challenges, Topical Conference Proceedings, NewOrleans, La., Mar. 10-14, 2002; Hickner et. al., Proceedings of theSymposium on Proton Conducting Membrane Fuel Cells III, 202^(nd) meetingof the ECS, Salt Lake City, Utah, Oct. 20-24, 2002]

Prior work [J. Kerres, J. New Mater. Electrochem. Syst. 5, 2, 2002, Y.S. Kim et al. The Electrochemical Society Meeting Abstract, vol.2003-2,Orlando, Fla., Oct. 12-16, 2003] indicated that relative performanceloss, decreasing current density with time at a fixed voltage, wascaused by interfacial incompatibility between the hydrocarbon basedmembranes (for example, sulfonated polyetherether ketone or sulfonatedpolyether sulfone) and Nafion®-bonded electrodes. Furthermore, a priorreport [Y. S. Kim et al. The Electrochemical Society Meeting Abstract,vol.2004-1, San Antonio, Tex., May 9-14, 2004] states that higherinterfacial resistance at the membrane/electrode interface is attributedto different water-swelling characteristics of the membrane andNafion®-bonded electrodes. The difference in water swelling ratiosgenerates mechanical stress at the membrane/electrode interface as amembrane-electrode assembly goes from the dry fabricated state to ahydrated state under fuel cell operating conditions. Generally, it isknown in the art that traditional hydrocarbon-based copolymers exhibithigher water uptakes than perfluorinated Nafion® membranes havingsimilar proton conductivity.

The core of a traditional PEMFC is referred to as a membrane-electrodeassembly (MEA), and includes: an anode catalyst layer, a polymerelectrolyte membrane (PEM), and a cathode catalyst layer. MEAs aretypically prepared either by direct painting of catalyst ink onto a PEMor by hot pressing catalyst layers to the PEM.

Method of Preparation with Corresponding Example

The present invention involves composite membranes; either single sidedor double sided, to include single or multiple layers for use incomposite MEAs and a method of preparing such MEAs. In the embodimentsof the invention presented here, single layer composites, both singlesided and double sided were demonstrated.

Referring now to FIG. 1, a flow chart of two embodiments (single anddouble sided) of a method of preparation for a novel composite membrane:

In Step 10, a base polymer is selected for a base membrane and anelectrode composition is selected for an electrode. Both the basepolymer and electrode composition are chosen based on desired propertiesof the resultant fuel cell MEA. For the base polymer, properties such asconductivity, permeability, electro-osmotic drag coefficient, cost, andthermal mechanical or chemical robustness are considered. The electrodecomposition is chosen primarily on electrochemical activity and cost,although other issues such as mass transport, ionic and electronicconductivity, catalyst loading and catalyst support composition are alsoconsidered. As evidenced by the present invention, the focus should bein choosing a polymeric material with optimized properties and matchingit with electrodes with optimized properties. As for many conventionalfuel cells, the best membrane materials are poorly matched withoptimized electrodes.

The novel aspect of the present invention is the use of an intermediatelayer(s) to improve adhesion and provide an ability to “tune” thephysical property performance between membranes and electrodes ofoptimized composition. An exciting additional benefit is the ability tocontrol the water content of a membrane under operating conditions byusing materials with differing electro-osmotic drag coefficients asasymmetric composites. The novelty of this approach is based on choosingand applying coating layers to base polymer membranes. For oneembodiment of the present invention, a wholly aromatic sulfonatedpolyarylene ether sulfone (BPSH -35—35% disulfonated, biphenyl sulfonefor the monomers used in the polymerization) was chosen based onincreased proton conductivity to methanol permeability ratio compared toNafion® (Kim et. al, J. Memb. Sci., 243, 317 (2004)). DMFC electrodes,containing Nafion® and either unsupported Pt or PtRu, were selected inthis embodiment based on displayed optimized performance.

The choice for a base membrane material may be very broad, althoughspecific classes of polymers such as those based on wholly aromatic orperfluorinated backbones, for stability, and containing strongly acidicentities (such as sulfonic acid), for conductivity are preferred. Thechoice of electrode materials is equally broad, although two commoncomponents are a catalyst and a proton-conducting polymer, such asNafion®.

In Step 20, materials for the property-tuning coating layer(s) arechosen. The choice of material for a property-tuning coating layer(s) isbased on compatibility with the base membrane and the electrodes. Whenconsidering multiple layer composites, compatibility with contactinglayers is required, however consideration of contact with the basemembrane and electrodes is not required. Compatibility in these systemsis defined as the ability of the coating system to adhere to andmaintain ionic conductivity between the coating layer and either theelectrodes or base membrane, and includes contacting layers in the caseof multilayer composites.

In Step 30, the choice of solvent is based on the interaction of thesolvent with the property-tuning coating layer(s) material and the basemembrane. Solvents, either single component or multi-component, areselected based on solubility characteristics with the coating polymer(s)and base membrane material. Ideally, solvents that allow full solubilityof the selected polymers (coating and base materials) are employed.However, other solvents that either lead to significant swelling or tothe formation of small particle size polymer dispersions might bepractical if the result is robust polymer layers with good adhesion.

For one embodiment of the present invention demonstrated here, partiallyfluorinated polysulfone (6FCN -35 or 6F-30) copolymer was chosen as theproperty-tuning coating, because it is highly compatible with the highperformance electrode material. Dimethyl acetamide (DMAc) was usedbecause it is a known and preferred solvent for both the base membrane(BPSH-35) and the coating polymer (6FCN-35 or 6F-30).

In Step 40, the base membrane is prepared for application ofproperty-tuning coating layers. The base membrane can be obtainedcommercially or by casting or extrusion from a polymer solution or melt.The preparation steps may include: treating with peroxide, de-ionizedwater and/or acid to remove impurities; swelling with appropriatesolvents; or putting the membrane into or on an apparatus where coatinglayers can be applied.

In the example presented here, BPSH -35 in the sodium salt form was castfrom a 5% (by weight) polymer solution of DMAc. This membrane wasconverted to the proton form by boiling in acid and boiled in deionizedwater three times to remove free acid. The hydrated membrane was thenplaced on a vacuum table (a porous metal plate, with vacuum pulled fromthe backside, and held at a constant controlled temperature, most often75° C.). A mask was placed over the membrane that exposed the surfacefor coating, but sealed the vacuum being pulled by the porous plate.This system restrained expansion of the base membrane during coating andallowed accelerated removal of solvent from the coating solution as itwas applied (Step 50).

In Step 50, a property-tuning coating layer solution is prepared usingthe appropriate polymer-solvent combinations determined in Step 20.Processing parameters of note include: stirring rate, time, temperatureand pressure, concentration and filtration of the resultant polymersolution (or dispersion).

In the present embodiment, partially fluorinated polysulfone (either6FCN-35 or 6F-30) copolymer was prepared by dissolving the polymer inDMAc (5% by weight) and filtering the dissolved solution through asyringe filter. A sub-step in preparation includes passing the 6FCN or6F polymer solution through a 0.45-micron filter to remove undissolvedpolymer or salt impurity that could adversely affected the coatinglayers by reducing coating integrity or adhesion.

Next, in Step 60, the prepared property-tuning coating solution (ordispersion) is applied to the base membrane in order to form a singlesided composite. The polymer may be applied by a number of methods suchas painting, spraying, dip coating, slot coating, knife coating, spincoating or other coating techniques (some of which can allow for adouble sided composite to be made in a single step, although this is notrequired). The conditions of the coating process, such as thickness ofcoats or time between coats for multiple coat process, temperature ofsystem, inert gas flow rate over sample, etc. can play a role indetermining the resultant composite membrane properties.

For the embodiment of the invention presented here, a hand paintingtechnique was used for painting the coating solution onto one side ofthe base membrane on a vacuum table (conditions used were ˜1 inchmercury absolute pressure and 80° C.). Six coats were applied to themembrane by a #2 camel hair brush, with approximately 1 minute intervalsin-between each application. After the final coat, the composite washeld at 80° C. on the vacuum table for at least 30 minutes to removemost of the DMAc by evaporation, resulting in a coating thicknessapproximately 10 microns thick. For the single sided 6F compositesreported here, the composite membranes were complete with this singlestep. For the double-sided 6FCN composites, the composite membranes werecooled, removed from the vacuum table, and a coating layer was appliedto the second side of the membrane following Steps 40, 50 and 60 to thesecond side of the base membrane. Note that multiple layer compositesmay be produced by repeating Steps 40, 50 and 60 with different polymercoating layers.

In Step 70, the composite membrane is prepared for application ofcatalyst layers. The preparation of composite membranes for MEAproduction may involve a number of steps known to those skilled in theart, such as acidification (if the membrane is in a salt rather thanproton form), drying of the membrane, removing impurities from themembrane, solvent exchange, hydrating the membrane, and/or putting themembrane in an apparatus or process that allows catalyst layerattachment.

For the embodiment of the invention presented here, the completedcomposite membrane was removed from the vacuum table and placed indeionized water at room temperature for 12 hours to re-hydrate themembrane and any residual solvent. The composite was placed back on thevacuum table for application of the electrodes.

In Step 80, catalyst suspensions are prepared as commonly known to thoseskilled in the art and applied to the single or double sided composite.For symmetric membranes, the cathode and anode may be applied to eitherside of the membrane.

However, the orientation of single sided composites or asymmetricdouble-sided composites is important because of water transport issues.In polymer electrolyte fuel cells, protons travel through the polymerelectrolyte (from the anode to the cathode) in order to complete theelectrochemical reaction. Electro-osmotic drag, the transport of waterdue to the flow of protons, occurs in these materials and the extent ofdrag, the electro-osmotic drag coefficient or number of water moleculescarried, depends on the material.

Therefore, when materials with different electro-osmotic dragcoefficients are put into contact with each other, such as in the caseof this invention, water can be preferentially held in or kept out ofthe membrane under operating conditions. Water in the membrane isimportant because hydration level impacts critical performance factorssuch as conductivity, permeability and mechanical properties.

In this embodiment of the present invention, the example compositemembranes discussed here were placed on a vacuum table. A mask wasplaced over the membrane exposing only the coated surface forapplication of catalyst layers. The cathode ink was applied to one sideof the composite; the membrane was cooled, removed from vacuum andflipped over. The vacuum was reapplied and the anode ink was applied.

Finally, in Step 90, the resultant MEA is assembled into a fuel cell.

Single sided composites or asymmetric double-sided composites may beutilized in order to dynamically control hydration or dehydration underfuel cell operating conditions. Double sided composites allow for thebase membrane to provide the primary properties to the fuel cellelectrolyte while coating layers provide better adhesion to theelectrodes and therefore better performance and durability than the basemembranes alone provide.

EXAMPLE

Two standard MEAs (non-composite MEAs for comparison purposes) wereprepared, one using a Nafion® 1135 membrane (90 μm thick) and the otherusing a BPSH (sulfonated poly(arylene ether) sulfone) membrane (65 μmthick). Standard catalyst inks using unsupported platinum andplatinum-ruthenium catalyst were applied to both using a direct paintingtechnique. To prepare the catalyst ink mixtures, a 5% Nafion® dispersion(1100 equivalent weight made by Solution Technology, Inc.) was added tothe water-wetted catalyst.

The anode ink composition was 86 wt % 1:1 platinum-ruthenium (made byJohnson Matthey, Plc.) and 14 wt % Nafion®, and the cathode inkcomposition was 90 wt % platinum black (Johnson Matthey, Plc.) and 10 wt% Nafion®. Catalyst inks were mixed by sonication for about 90 secondsand then directly transferred to a pre-dried membrane by direct paintingat 75° C. The anode and cathode catalyst loading were approximately 10and 6 mg/cm², respectively. Single and double side hydrophobic carboncloths (made by E-TEK, Inc.) were used as anode and cathode gasdiffusion layers, respectively. The two MEAs were then subjected to atest at 80° C. under DMFC operating conditions. (i.e., 0.5 M methanolfeed at the anode, air flow at the cathode with humidification butwithout backpressure)

FIGS. 2 a and 2 b show the polarization curves of a Nafion® and a BPSHMEA at three points in time under DMFC operating conditions: initialperformance, after 200 hours, and after 500 hours. The initialperformance of the BPSH MEA was significantly superior to theperformance of the Nafion® MEA, due to lower methanol crossover andcomparable cell resistance. However, the BPSH MEA exhibited a greaterdecay in performance over time than the Nafion® MEA. After the 500 hlife test, the current density losses for the BPSH and Nafion® MEAs were70 and 35 mA/cm² at 0.5 V, respectively, resulting in comparableperformance between the two MEAs after 500 hours.

In the case of the BPSH MEA, the greater drop in performance wasaccompanied by an increase in high frequency resistance, as opposed tothe high frequency resistance of the Nafion® MEA that remainedrelatively constant over the 500 hour test. The performance loss andcell resistance increase for the BPSH MEA is attributed to interfacialresistance attributed to delamination between the electrodes and themembrane.

FIGS. 3 a and 3 b show the performance of the same two polymers inH₂/air fuel cells. Improved reaction kinetics and less reactantcrossover result in increased efficiency and higher power output.However, trends in decreasing performance and increasing high frequencyresistance (with the BPSH MEA) mirror the results of DMFC testing inFIGS. 2 a and 2 b, suggest that similar interfacial resistancephenomenon are at work regardless of whether hydrogen or methanol isused as the fuel.

The solution provided by the present invention is to reduce the role ofinterfacial phenomenon, thereby improving performance and durability.This is accomplished by employing a multi-component membrane systemcomprising multiple coating layers, such as that shown in FIGS. 4 a and4 b. The role of coating layers in this form is to reduce the mechanicalstress between the electrode layers and the proton exchange membrane,thereby improving interfacial compatibility and performance, and/or as away to control hydration level under operating conditions.

In one embodiment of the present invention, BPSH with a 35% degree ofdisulfonation (BPSH-35) (65 μm thick) was used as the polymerelectrolyte membrane and hexafluoro bisphenol A based poly(arylene etherbenzonitrile) with a 35% degree of disulfonation (hereinafter 6FCN-35)was used as the coating material. Dimethylacetamide (hereinafter DMAc)was used as the co-solvent.

The 6FCN-35 copolymer was dissolved in DMAc (5 wt %) and coated ontoeach side of the dry BPSH membrane using a hand brush coating technique.The coated BPSH membrane was then dried on a vacuum table at 80° C. for3 hours. The coating thickness of the 6FCN-35 layer was then measuredusing a micrometer. Each coating layer thickness was determined to beabout 10 μm. The coated BPSH membrane was then immersed in deionizedwater at 25° C. for a minimum of 12 hours to remove the residual DMAcsolvent. No delamination between membrane and coating layer wasobserved. The catalyst layers were applied by direct painting in thesame manner as described previously for catalyst layer application onuncoated membranes.

FIG. 5 shows the direct methanol and hydrogen fuel cell performance of a6FCN-35 coated BPSH-35 MEA for the above embodiment of the invention.The performance of the 6FCN-35 coated BPSH-35 MEA is compared to anuncoated BPSH-35 MEA and a Nafion® MEA by comparing the results shown inFIG. 5 to the results in FIGS. 2 a/ 2 b and 3 a/ 3 b.

The results show that when compared to the uncoated system, the 6FCN-35coated BPSH MEA has a slightly decreased initial performance, due to theadditional resistance of the coating layers. The increased resistance ofthe coating layers is an artifact of the increased MEA thickness (˜85 μmfor the coated membrane vs ˜65 μm for the uncoated membrane) and can bemitigated by using a thinner substrate or coating layers, therebyresulting in a system without increased resistance and having comparableperformance to the uncoated system.

The observed long-term performance of these systems shows the benefitsprovided by the present invention. The long term performance of thecoated system is stable, while that of the uncoated system is notstable, as witnessed by the changing polarization curves with time forthe uncoated system (FIGS. 2 a/ 2 b and 3 a/ 3 b), compared to thestable response of the 6FCN-35 coated system (FIG. 5). The changing fuelcell resistances of these MEAs, also shown in FIGS. 2 a/ 2 b, 3 a/ 3 b,and 5, elucidate the long-term performance changes.

The uncoated BPSH-35 system (FIGS. 1 and 2) exhibits a significantincrease in cell resistance over time, while the 6FCN-35 coated system(FIG. 5) exhibits a significant decrease in cell resistance over time.The increasing cell resistance over time in the uncoated system isattributed to poor interfacial adhesion and delamination of the membraneand electrodes over time. The decreasing cell resistance over time inthe coated system is attributed to morphological changes in the polymerand to the effects of polymer hydration over time. A similar decreasewas expected in the uncoated BPSH-35 system, suggesting that interfaciallosses in this system are larger than decreases in resistance due tomorphological changes and hydration. The stable performance anddecreasing resistance of the coated 6FCN-35 system proves that losses inperformance due to interfacial phenomenon are addressed by the presentinvention.

The benefits of hydrogen and methanol fuel cell performance for the6FCN-35 coated system in FIG. 5 can also be interpreted in comparison tothe results obtained for a traditional fuel cell material like Nafion®1135, as shown in FIGS. 2 a/ 2 b and 3 a/ 3 b. While the membranethicknesses of the two samples were nearly identical (˜85 μm for thecoated membrane vs ˜90 μm for Nafion® 1135), the performancecharacteristics of Nafion® 1135 were more stable than the uncoatedBPSH-35 membrane (also shown in FIGS. 1 and 2), which showedsignificantly faster degradation than the 6FCN-35 coated membrane shownin FIG. 5. Cell resistance of the Nafion® 1135 sample remained stableover time; thus, the membrane-electrode interface is relatively stablecompared to the uncoated BPSH-35 system.

For DMFCs, the improved performance of the non-Nafion® based systems isreflected in the lower open circuit methanol crossover rates. The6FCN-35 coated system (˜85 μm) had the lowest crossover, at 33 mA/cm²,the uncoated BPSH-35 sample (˜65 μm) had a higher crossover, at 75mA/cm², and the Neon® 1135 (˜90 μm) sample exhibited the highestcrossover, at 95mA/cm², for 0.5M methanol feed at 80° C. Methanolcrossover in these systems adversely affects both fuel utilization andcell performance, thus, the lower the crossover rate, the better theperformance.

The example above was chosen with regards to compatibility of the6FCN-35 polymer with both the BPSH-35 base polymer and the electrodelayers, and the properties of the base BPSH-35 material. An earlierstudy found that 6FCN-35 (hexafluoro bisphenol A based poly(aryleneether benzonitrile) with a 35% degree of disulfonation) exhibitedinterfacial compatibility with the electrodes [Y. S. Kim, M. J. Sumner,W. L. Harrison, J. S. Riffle, J. E. McGrath, B. S. Pivovar, J.Electrochem. Soc, 2004, 151, A2150]. It was also determined that BPSH-35was a promising material for fuel cells, particularly direct methanolfuel cells, although issues of durability were a concern [Y. Kim et.al., Extended Abstracts of the 2004 Fuel Cell Seminar, San Antonio,Tex., Nov. 1-4, 2004]. The solubility of these copolymers in a commonsolvent (in this case DMAc) suggests good compatibility between thepolymeric materials.

Although the above embodiment was prepared with DMAc, any solvent foreither polymer that is known to form a polymer solution from whichmembranes with good mechanical properties may be used. The choice ofsolvent and processing conditions are important aspects of compositecompatibility in MEAs. It is recognized that a wide variety of solventsor processing conditions (such as temperature, method of application,application of a restraint, application of a vacuum, etc.) may be usedin applying a multilayer composite approach, and that these factors canplay an important role in resultant properties. In the presentinvention, the removal of residual solvent was necessary to preventcontamination of the catalyst.

While the above described embodiment of the invention was produced byapplication of coating layers through hand brush coating, it isrecognized that a variety of coating methods may be employed, such asdip coating, spin coating, spray coating, slot coating, and knife ordoctor blade coating. It is also recognized that the present inventionmay be used for creating multiple coating layers that may be necessaryor advantageous.

The improvement of interfacial compatibility is in part attributed to adecreased water uptake of the 6FCN-35 coating layer compared to BPSH-35membrane, thereby more closely matching the volume change upon hydrationof the Nafion® electrodes and leading to less mechanical stress at theelectrode-membrane interface.

Another polymer, BPSH-30 (the same polymer backbone as BPSH-35 with a30% disulfonation level rather than 35%), with lower water uptake wasalso investigated as a coating layer. Table I shows the results oftesting a BPSH-35 membrane without a coating layer and with a coatinglayer of either BPSH-30 or 6FCN-35, prepared by using the method of thepresent invention. While the initial DMFC performance of the coatedmembrane systems at 0.5V is decreased for the coated systems, due to theincreased resistance of the coating layers, the stability in performanceof either system is much more stable over time. These resultsdemonstrate that either coating layer presents an improvement indurability over an uncoated system, although thinner materials withlower overall resistance are necessary for attaining equivalent initialperformance.

TABLE 1 Performance of Uncoated and Coated BPSH-35 MEAs Initial DMFCDuration current Current Change of cell density at density at in CellMem- Coating operation 0.5 V ^(a) 0.5 V lost ^(a) resistance brane layer(hours) (mA/cm²) (mA/cm²) (mΩ cm²) BPSH- None 200 212 23 29 35 700 68 53BPSH- 200 142 7 5 30 6FCN- 200 182 0 −20 35 700 −2 −28 2000 6 −30 ^(a)current density was measured from current-voltage polarization curve.^(b) cell resistance was measured from in-situ cell high frequencyresistance response.

MEA Design and Implementation

The coated MEA systems discussed to this point have been symmetric andoperated under conditions in which differences in properties of theindividual components have played little role on performance, and havebeen presented for the manner they improve performance and durability.Composite membranes also allow the potential of tuning transportproperties to produce composite materials that behave or function, inapplication, in ways that single component systems cannot.

For example, the use of composite membranes can retain water or preventexcessive hydration within the polymeric materials under operatingconditions. These issues are relevant for conductivity at low relativehumidity, an important concern for many fuel cell applications includinghigh temperature operation (>80° C.), or for the potential utilizationof materials that lack sufficient mechanical properties at high levelsof hydration, but may be of interest for their transport properties suchas high conductivity.

In polymer electrolyte fuel cells, protons travel through the polymerelectrolyte (from the anode to the cathode) in order to complete theelectrochemical reaction. Electro-osmotic drag, the transport of waterdue to the flow of protons, occurs in these materials and the extent ofdrag, the electro-osmotic drag coefficient or number of water moleculescarried, depends on the material. Therefore, when materials withdifferent electro-osmotic drag coefficients are put into contact witheach other, such as in the case of this invention, water can bepreferentially held in or kept out of the membrane under operatingconditions. This is illustrated in FIGS. 6 a, 6 b, and 6 c. FIG. 6 ashows a traditional MEA where a high electro-osmotic drag membrane (e.g.Nafion®) was sandwiched with anode and cathode electrodes). FIGS. 6 band 6 c show single sided composites where a low electro-osmotic dragmaterial is used as a coating layer on a high electro-osmotic drag basemembrane. In FIG. 6 b, the low electro-osmotic drag material is on thecathode side of the cell, resulting in a situation where water carriedfrom the anode, due to electro-osmosis, is held within the basemembrane. This geometry is useful for fuel cells at elevated temperature(>100° C.) where membrane dehydration is an issue. In FIG. 6 c, the lowelectro-osmotic drag material is on the anode side of the cell,resulting in a low hydration level in the base membrane because water iscarried away from the interface faster than it can travel through thecoating layer. This geometry is useful for fuel cells where mechanicalproperties of the base membrane are poor at high levels of hydration orwhere cathode flooding restricts fuel cell performance.

Water in the membrane is a critical issue in fuel cell performance, asthe polymer electrolytes require water to conduct, however somematerials like those of high ionic content can dissolve in the presenceof too much water. Additionally, system implications such as energylosses due to humidification or airflow rates are highly dependent onthe water balance within the system. The use of either symmetric orasymmetric composite membranes can enable improved properties andperformance depending on operating conditions. The key is tuning factorssuch as the water diffusion coefficient or the electro-osmotic dragcoefficient of the multiple layers employed so that water is retained orexpelled under the operating conditions of the device.

A single sided composite membrane is illustrated in FIG. 4 b. Unlike thesymmetric multilayer membrane shown in FIG. 4 a, the asymmetric membranehas a coating layer on only a single side of the membrane and can beimplemented in a fuel cell by placing the coated side of the membranetoward either the anode or cathode layer, although only the cathodeorientation is shown in FIG. 4 b. The orientation of single sidedcomposites or asymmetric double-sided composites is important because ofwater transport issues.

FIGS. 7 a and 7 b demonstrate an advantage of a single sided compositemembrane compared to a single component membrane under low relativehumidity (elevated temperature) operating conditions. In this embodimentof the invention, fluorinated poly(arylene ether sulfone) (6F-30) wasused as the coating material for the cathode side of a Nafion® membrane.In this situation, the electro-osmotic drag coefficient for 6F-30 is 1.4water molecules per proton, compared to 3.3 water molecules per protonfor Nafion®. For the operating conditions investigated, the 6F-30 coatedNafion® cathode shows minimal performance loss with increasing celltemperature compared to a Nafion® MEA without the coating. The improvedperformance of the coated Nafion® under high temperature fuel celloperation is due to a better ability to maintain cell conductivityafforded by the water transport properties of the MEA and cell geometry.This is reflected in the high frequency resistance measurements shown inFIGS. 7 a and 7 b. The cell resistance for the coated Nafion® cathodeincreased only slightly with temperature, compared to the un-coatedNafion® control that showed a significant increase in resistance withincreasing cell temperature (decreasing cathode relative humidity). Notethat this was especially true at low current densities, where watergeneration at the cathode was small. This result shows the ability ofthe composite membrane system to maintain conductivity under conditionsin which a single component membrane does not.

The foregoing description of the invention has been presented forpurposes of illustration and description and is not intended to beexhaustive or to limit the invention to the precise form disclosed, andobviously many modifications and variations are possible in light of theabove teaching.

The embodiments were chosen and described in order to best explain theprinciples of the invention and its practical application to therebyenable others skilled in the art to best utilize the invention invarious embodiments and with various modifications as are suited to theparticular use contemplated. It is intended that the scope of theinvention be defined by the claims appended hereto.

1-3. (canceled)
 4. A polymer electrolyte membrane assembly, comprising,a. a base membrane to provide high proton conductivity, thermalstability, and low reactant permeability, said base membrane comprisinga base polymer comprising disulfonated biphenyl poly(arylene ethersulfone), b. an electrode composition to provide electrochemicalreactivity, reactant access, product removal, ionic and electronicconductivity, said electrode composition comprising a catalyst and aproton-conducting polymer, wherein the proton-conducting polymer isdifferent than the base polymer; and c. a property-tuning coating layercomposition connectively attached to said base membrane and saidelectrode to provide proton conductivity and material compatibility withsaid base membrane and said electrode, said property-tuning coatinglayer composition being selected from the group consisting of adisulfonated hexafluoro bisphenol A based poly(arylene etherbenzonitrile) and a fluorinated poly(arylene ether sulfone). 5-7.(canceled)
 8. The polymer electrolyte membrane assembly of claim 4,wherein said catalyst is selected from the group consisting of platinum,platinum-ruthenium, platinum black, and combinations thereof.
 9. Thepolymer electrolyte membrane assembly of claim 4, wherein saiddisulfonated biphenyl poly(arylene ether sulfone) comprises a 35% degreeof disulfonation.